[calc] How display negative vals. in red?

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There are predefined currency formats for that. None in the plain "numbers" category.

The full format spec (if you use the manual format codes near the bottom of the dialog) can have 4 sections separated by semicolon. With all 4 in use, the first is for positive values, second for negative, third for zero, and fourth for text. Often you don't need more than positive and negative (zero will then fall back to the format for positive, and text is rendered verbatim).

Colors can be specified by bracketed hex RGB values or color names. Assuming that you use period (dot) for decimal point: Try 0.00;[red]0.00